1. Field of the Invention
The present invention relates to a system for preventing belt slip of a belt-type continuously variable transmission.
2. Description of the Prior Art
Conventionally, as a continuously variable transmission for a vehicle, there is, for example, a belt-type continuously variable transmission (hereinafter referred to as a “belt CVT”) with a V-belt.
In this belt CVT, a transmission mechanism is constructed by winding the V-belt around pulleys comprised of a primary pulley connected to an engine side and a secondary pulley connected to an axle side, and groove widths of the primary pulley and the secondary pulley are variably controlled by hydraulic pressure.
A thrust of the pulley is determined according to an input torque and a gear ratio and converted into a hydraulic pressure on the basis of predetermined values such as pressure-receiving areas of the secondary pulley and the primary pulley, and this hydraulic pressure is supplied to the transmission mechanism as a target line pressure.
The primary pulley and the secondary pulley are respectively provided with a first cylinder chamber and a second cylinder chamber, and a primary pressure obtained by regulating line pressure and a secondary pressure obtained by regulating line pressure are respectively supplied to the first cylinder chamber and the second cylinder chamber. The groove widths of the primary pulley and the secondary pulley are changed by hydraulic pressure supplied to the respective cylinder chambers during running of the vehicle, and gear ratio is continuously changed according to a contact radius ratio (pulley ratio) between the V-belt and the respective pulleys.
While line pressure is controlled to be held to a minimum pressure during idling of the engine, a torque-down control to restrain output torque of the engine is performed so that the transmission mechanism receives no excessive torque exceeding a limit value of an allowable input torque of the belt CVT. A torque-down amount by this torque-down control is usually determined from a throttle valve opening and an engine speed and is set so as to reduce according to the increase of the engine speed and to increase according to the increase of the throttle valve opening.
In such a belt CVT, when the vehicle is once stopped by removal of a driver's foot from the accelerator pedal and braking during running on a upward slope in a forward “D” range and started again in the “D” range, the vehicle is slightly moved backward by the removal of the driver's foot from a brake pedal, and then a reverse torque is applied to an output shaft of the belt CVT, and a reverse rotation is generated in the pulleys.
Even if the pulley ratio, the input torque, an input rotation speed or the secondary pressure is the same when the pulleys are reversely rotated, balance in hydraulic pressure between the primary pressure and the secondary pressure is lost, the primary pressure is especially reduced by half, and torque capacity of the primary pulley is reduced, so that belt slip may be generated. However, no control to prevent this disruption of the balance of hydraulic pressure is conventionally performed.
In case a control to hold the line pressure to a minimum pressure during idling by removal of the driver's foot is adopted, the hydraulic pressure is maintained low under a condition that the reverse rotation of the pulley is generated.
This reverse rotation of the pulleys is generated also when the vehicle is started again in an “R” range after it is stopped once while running backward in the “R” range on a down slope, whereby the same problem occurs.
That is, the reverse rotation of the pulleys discussed herein means a phenomenon that the pulleys are rotated reversely with respect to a direction of a normal rotation of the pulleys (forward direction in the “D” range or backward direction in the “R” range) to be supposed in a currently selected range position. The reverse rotation of the pulleys is used in this meaning hereinafter.